feat(theme): darker text + buttons in light themes (revert modal backing)

Replace the light-theme modal backing (19e3138, now reverted) with a root-cause
fix: darken the light themes' UI elements for contrast.
- Buttons: the light-theme ImGuiCol_Button overlays were barely-there grey
  (5/8/12% black); bump to 13/20/28% so buttons read as buttons on light
  surfaces (one edit in ApplyColorThemeToImGui covers all light skins).
- Text: raise --on-surface-medium (60-72% -> 86%) and --on-surface-disabled
  (38-40% -> 52%) across the light skins (light, color-pop-light, dune, marble,
  iridescent) — the dominant secondary/label text was too faint. Base
  --on-surface was already near-black, left as-is.
The active theme is TOML-skin-driven, so the code GetMaterialLightTheme() is not
the live source; edits are in the skin TOMLs + the ImGui-style mapping.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/themes/dune.toml

@@ -20,8 +20,8 @@ elevation = { --elevation-0 = "#FDF8F0", --elevation-1 = "#F5EDE0", --elevation-
 --on-secondary = "#FFFFFF"
 --on-background = "#3A2E22"
 --on-surface = "#3A2E22"
---on-surface-medium = "rgba(58,46,34,0.68)"
---on-surface-disabled = "rgba(58,46,34,0.38)"
+--on-surface-medium = "rgba(58,46,34,0.86)"
+--on-surface-disabled = "rgba(58,46,34,0.52)"
 --error = "#A0524A"
 --on-error = "#FFFFFF"
 --success = "#6A8A5C"